This application relates generally to fuel tank inerting systems, and specifically to catalytically inerted fuel tank systems.
Fuel tanks can contain potentially combustible combinations of oxygen, fuel vapors, and ignition sources. In order to prevent combustion in aircraft fuel tanks, commercial aviation regulations require actively managing the risk of explosion in ullage of fuel tanks; this is typically accomplished by decreasing the oxygen partial pressure in the ullage in fuel tanks to less than 12%, or less than 9% for military vehicles. Conventional fuel tank inerting systems use air separation modules that separate out oxygen and humidity from incoming bleed air, generating nitrogen enriched air (inert air) to fill the ullage of the fuel tank.
An alternative method is to use catalytic oxidation to produce an inert air stream that does not require use of pressurized bleed air. Catalytic oxidation includes burning of fuel with a catalyst with air to produce inert gas. With proper thermal and humidity management, the resulting inert gas can be introduced into the ullage of the fuel tank as a fuel tank inerting method.
Catalytic oxidation produces inert gas (mostly nitrogen), but also produces byproducts water vapor and carbon dioxide. When heavy fuel fractions are used in catalytic oxidation, about 15% of the produced inert gas is carbon dioxide. The remainder of inert gas produced in catalytic oxidation is mostly nitrogen and water vapor, which can be removed prior to introduction to the fuel tanks. However, when the inert gas is introduced to the ullage of the fuel tank, the solubility of carbon dioxide in kerosene-based fuel is much greater than oxygen or nitrogen, and the solubility of carbon dioxide decreases as the fuel temperature rises due to the fuel's use as a heat sink in aircraft engines. This creates a unique problem with dissolved carbon dioxide.
In aircraft, fuel travels from the fuel tanks to the engines. A commercial aircraft typically contains at least three fuel tanks: one or more are located in each wing in addition to one in the fuselage. Along the fuel flow path from the fuel tanks to the engine, fuel is used for temperature regulation throughout the aircraft engines. If high levels of carbon dioxide are dissolved in fuel, cavitation (bubbling) due to carbon dioxide dissolved in the fuel can occur at undesirable locations within the aircraft engine fuel system.